Molecular mechanism of anesthetic-induced depression of myocardial contraction

Cardiac health, type I collagen, and aging in the oim/oim mouse model of osteogenesis imperfecta and a cohort of adults with OI

Osteogenesis imperfecta (OI) is a heritable connective tissue disorder with marked skeletal fragility and increased recognition as a pleiotropic type I collagenopathy. The impact of OI-causing gene variants on cardiac health and lifespan is just beginning to be understood. To begin to investigate cardiac manifestations of OI-causing type I collagen variants, we utilized the osteogenesis imperfecta murine (oim/oim) model to examine survival with increased age, as well as cardiac function and collagen expression at 4 and 18 mo of age. We determined male oim/oim mice had 50% decreased survival by 18 mo of age compared with wild-type (WT) littermates. Cardiac magnetic resonance imaging and echocardiography revealed 18-mo-old male oim/oim mice had increased left ventricular end-diastolic and end-systolic volumes concomitant with decreased function, as well as the presence of aortic stenosis in a subset of 4- and 18-mo-old male oim/oim mice compared with WT littermates. Female oim/oim survival and cardiac function were equivalent to their WT counterparts. Cardiac evaluations of an adult patient cohort with OI corroborated increased incidences of valvular dysfunction in the patient population with OI, with much of the male cohort also presenting with altered left ventricular function. Little is known concerning the impact of OI-causing variants on patient cardiac health and the influence of sex and age. Using an OI mouse model, we determined that 18-mo-old male oim/oim mice have cardiac dysfunction with decreased lifespan, confirming the need for further investigations to understand pleiotropic extraskeletal manifestations and disease progression in osteogenesis imperfecta.NEW & NOTEWORTHY The heritable skeletal dysplasia, osteogenesis imperfecta (OI), recently recognized as a pleiotropic collagenopathy, shows growing evidence of cardiac involvement impacting lifespan. Evaluating cardiac function (magnetic resonance imaging and echocardiography) using an OI mouse model revealed increased left ventricular end-diastolic and end-systolic volumes concomitant with decreased function and reduced survival in 18-mo-old male OI mice. Additional cardiac evaluations of an adult patient cohort with OI corroborated increased incidences of valvular dysfunction in the patient population with OI.

The Role of GABA Receptors in Anesthesia and Sedation: An Updated Review

GABA (γ-aminobutyric acid) receptors are constituents of many inhibitory synapses within the central nervous system. They are formed by 5 subunits out of 19 various subunits: α1-6, β1-3, γ1-3, δ, ε, θ, π, and ρ1-3. Two main subtypes of GABA receptors have been identified, namely GABAA and GABAB. The GABAA receptor (GABAAR) is formed by a variety of combinations of five subunits, although both α and β subunits must be included to produce a GABA-gated ion channel. Other subunits are γ, δ, ε, π, and ϴ. GABAAR has many isoforms, that dictate, among other properties, their differing affinities and conductance. Drugs acting on GABAAR form the cornerstone of anesthesia and sedation practice. Some such GABAAR agonists used in anesthesia practice are propofol, etomidate, methohexital, thiopental, isoflurane, sevoflurane, and desflurane. Ketamine, nitrous oxide, and xenon are not GABAR agonists and instead inhibit glutamate receptors-mainly NMDA receptors. Inspite of its many drawbacks such as pain in injection, quick and uncontrolled conversion from sedation to general anesthesia and dose-related cardiovascular depression, propofol remains the most popular GABAR agonist employed by anesthesia providers. In addition, being formulated in a lipid emulsion, contamination and bacterial growth is possible. Literature is rife with newer propofol formulations, aiming to address many of these drawbacks, and with some degree of success. A nonemulsion propofol formulation has been developed with cyclodextrins, which form inclusion complexes with drugs having lipophilic properties while maintaining aqueous solubility. Inhalational anesthetics are also GABA agonists. The binding sites are primarily located within α+/β- and β+/α- subunit interfaces, with residues in the α+/γ- interface. Isoflurane and sevoflurane might have slightly different binding sites providing unexpected degree of selectivity. Methoxyflurane has made a comeback in Europe for rapid provision of analgesia in the emergency departments. Penthrox (Galen, UK) is the special device designed for its administration. With better understanding of pharmacology of GABAAR agonists, newer sedative agents have been developed, which utilize "soft pharmacology," a term pertaining to agents that are rapidly metabolized into inactive metabolites after producing desired therapeutic effect(s). These newer "soft" GABAAR agonists have many properties of ideal sedative agents, as they can offer well-controlled, titratable activity and ultrashort action. Remimazolam, a modified midazolam and methoxycarbonyl-etomidate (MOC-etomidate), an ultrashort-acting etomidate analog are two such examples. Cyclopropyl methoxycarbonyl metomidate is another second-generation soft etomidate analog that has a greater potency and longer half-life than MOC-etomidate. Additionally, it might not cause adrenal axis suppression. Carboetomidate is another soft analog of etomidate with low affinity for 11β-hydroxylase and is, therefore, unlikely to have clinically significant adrenocortical suppressant effects. Alphaxalone, a GABAAR agonist, is recently formulated in combination with 7-sulfobutylether-β-cyclodextrin (SBECD), which has a low hypersensitivity profile.

A Comparison of Dobutamine, Norepinephrine, Vasopressin, and Hetastarch for the Treatment of Isoflurane-Induced Hypotension in Healthy, Normovolemic Dogs

Isoflurane is a commonly used inhalation anesthetic in species undergoing veterinary care that induces hypotension, impacting organ perfusion, making it imperative to minimize its occurrence or identify effective strategies for treating it. This study evaluated and compared the hemodynamic effects of DOB, NEP, VAS, and HES in twelve isoflurane-anesthetized Beagle dogs. The order of the first three treatments was randomized. HES was administered last. Data were collected before treatments (baseline) and after 10 min of a sustained MAP of <45 mmHg induced by a high end-tidal isoflurane concentration (T0). Once treatment was initiated and the target MAP was achieved (65 to 80 mmHg) or the maximum dose reached, data were collected after 15 min of stabilization (T1) and 15 min after (T2). A 15 min washout period with a MAP of ≥65 mmHg was allowed between treatments. The intravenous dosage regimens started and were increased by 50% every five minutes until the target MAP or maximum dose was reached. The dosages were as follows: DOB, 5-15 μg/kg/min; NEP, 0.1-2 μg/kg/min; VAS, 0.5-5 mU/kg/min; and HET, 6% 1-20 mL/kg/min. DOB improved CO, DO2, and VO2, but reduced SVR. VAS elevated SVR, but decreased CO, DO2, and VO2. HES minimally changed BP and mildly augmented CO, DO2, and VO2. These treatments failed to reach the target MAP. NEP increased the arterial BP, CO, MPAP, and PAWP, but reduced HR. Norepinephrine infusion at 0.44 ± 0.19 μg/kg/min was the most efficient therapy for correcting isoflurane-induced hypotension.

Evaluation of Electrical Cardiometry for Measuring Cardiac Output and Derived Hemodynamic Variables in Comparison with Lithium Dilution in Anesthetized Dogs

Numerous cardiac output (CO) technologies were developed to replace the 'gold standard' pulmonary artery thermodilution due to its invasiveness and the risks associated with it. Minimally invasive lithium dilution (LiD) shows excellent agreement with thermodilution and can be used as a reference standard in animals. This study evaluated CO via noninvasive electrical cardiometry (EC) and acquired hemodynamic variables against CO measured using LiD in six healthy, anesthetized dogs administered different treatments (dobutamine, esmolol, phenylephrine, and high-dose isoflurane) impacting CO values. These treatments were chosen to cause drastic variations in CO, so that fair comparisons between EC and LiD across a wide range of CO values (low, intermediate, and high) could be made. Statistical analysis included linear regression, Bland-Altman plots, Lin's concordance correlation coefficient (ρ_c), and polar plots. Values of p < 0.05 represented significance. Good agreement was observed between EC and LiD, but consistent underestimation was noted when the CO values were high. The good trending ability, ρ_c of 0.88, and low percentage error of ±31% signified EC's favorable performance. Other EC-acquired variables successfully tracked changes in CO measured using LiD. EC may be a pivotal hemodynamic tool for continuously monitoring circulatory changes, as well as guiding and treating cardiovascular anesthetic complications in clinical settings.

Mechanistic basis of propofol-induced disruption of kinesin processivity

Propofol is a widely used general anesthetic to induce and maintain anesthesia, and its effects are thought to occur through impact on the ligand-gated channels including the GABA_A receptor. Propofol also interacts with a large number of proteins including molecular motors and inhibits kinesin processivity, resulting in significant decrease in the run length for conventional kinesin-1 and kinesin-2. However, the molecular mechanism by which propofol achieves this outcome is not known. The structural transition in the kinesin neck-linker region is crucial for its processivity. In this study, we analyzed the effect of propofol and its fluorine derivative (fropofol) on the transition in the neck-linker region of kinesin. Propofol binds at two crucial surfaces in the leading head: one at the microtubule-binding interface and the other in the neck-linker region. We observed in both the cases the order-disorder transition of the neck-linker was disrupted and kinesin lost its signal for forward movement. In contrast, there was not an effect on the neck-linker transition with propofol binding at the trailing head. Free-energy calculations show that propofol at the microtubule-binding surface significantly reduces the microtubule-binding affinity of the kinesin head. While propofol makes pi-pi stacking and H-bond interactions with the propofol binding cavity, fropofol is unable to make a suitable interaction at this binding surface. Therefore, the binding affinity of fropofol is much lower compared to propofol. Hence, this study provides a mechanism by which propofol disrupts kinesin processivity and identifies transitions in the ATPase stepping cycle likely affected.

Critical Review and Meta-Analysis of Postoperative Sedation after Adult Cardiac Surgery: Dexmedetomidine Versus Propofol

OBJECTIVE

To evaluate reports from the published literature of all randomized clinical trials (RCT) comparing postoperative sedation with dexmedetomidine versus propofol in adult patients, after open cardiac surgery.

DESIGN

A computerized search on Medline, EMBASE, Web of Science, and Agency for Healthcare Research and Quality databases was completed through June 2020. Meta-analysis of all published RCT comparing dexmedetomidine versus propofol utilization in the postoperative phase, using the standard Preferred Reporting Items for Systematic Reviews and Meta-Analyses checklist.

SETTING

Assemblage and critical discussion of 11 RCTs comparing postoperative sedation from standard published reports from 2003 to 2019.

PARTICIPANTS

The study comprised 1,184 patients and analyzed critical discussion of time-based parameters (time to extubation, intensive care unit length of stay, and hospital length of stay) and nontime-dependent factors (delirium, bradycardia, and hypotension).

MEASUREMENTS AND MAIN RESULTS

Time to extubation was significantly reduced in the dexmedetomidine group (standardized mean difference [SMD] = -0.70, 95% confidence interval [CI] -0.98 to -0.42, p < 0.001); however, no difference in mechanical ventilation time was observed (SMD = -0.72, 95% CI -1.60 to 0.15, N.S.). Dexmedetomidine significantly reduced the intensive care unit length of stay (SMD = 0.23, 95% CI -1.06 to -0.16, p = 0.008), but this did not translate into a reduced hospital length of stay (SMD = -1.13, 95% CI -2.43 to 0.16, N.S). For nontime-dependent factors, incidence of delirium was unaffected between groups (odds ratio [OR]: 0.68, 95% CI 0.43-1.06, N.S), and higher rates of bradycardia (OR: 3.39, 95% CI: 1.20-9.55, p = 0.020) and hypotension (OR: 1.68, 95% CI 1.09-2.58, p = 0.017) were reported with propofol.

CONCLUSIONS

Despite the ICU time advantages afforded by dexmedetomidine over propofol, the former did not seem to contribute to an overall reduction in hospital length of stay or improvement in postoperative outcomes of heart valve surgery and CABG patients.

Sevoflurane prevents vulnerable plaque disruption in apolipoprotein E-knockout mice by increasing collagen deposition and inhibiting inflammation

BACKGROUND

Sevoflurane may reduce the occurrence of major adverse cardiovascular events (MACCEs) in surgical patients, although the mechanisms are poorly understood. We hypothesised that sevoflurane stabilises atherosclerotic plaques by inhibiting inflammation and enhancing prolyl-4-hydroxylase α1 (P4Hα1), the rate-limiting subunit for the P4H enzyme essential for collagen synthesis.

METHODS

We established a vulnerable arterial plaque model in apolipoprotein E-knockout mice (ApoE^-/-) fed a high-fat diet that underwent daily restraint/noise stress, with/without a single prior exposure to sevoflurane for 6 h (1-3%; n=30 per group). In vitro, smooth muscle cells (SMCs) were incubated with tumour necrosis factor-alpha in the presence/absence of sevoflurane. Immunohistochemistry, immunoblots, and mRNA concentrations were used to quantify the effect of sevoflurane on plaque formation, expression of inflammatory cytokines, P4Hα1, and collagen subtypes in atherosclerotic plaques or isolated SMCs.

RESULTS

In ApoE^-/- mice, inhalation of sevoflurane 1-3% for 6 h reduced the aortic plaque size by 8-29% in a dose-dependent manner, compared with control mice that underwent restraint stress alone (P<0.05); this was associated with reduced macrophage infiltration and lower lipid concentrations in plaques. Sevoflurane reduced gene transcription and protein expression levels of pro-inflammatory cytokines (≥69-75%; P<0.05) and matrix metalloproteinases (≥39-65%; P<0.05) at both gene transcription and protein levels, compared with controls. Sevoflurane dose dependently increased Types I and III collagen deposition through enhanced protein expression of P4Hα1, both in vivo and in vitro (0.7-3.3-fold change; P<0.05).

CONCLUSIONS

Sevoflurane dose dependently promotes plaque stabilisation and decreases the incidence of plaque disruption in ApoE^-/- mice by increasing collagen deposition and inhibiting inflammation. These mechanisms may contribute to sevoflurane reducing MACCE.

Lysine acetylation of F-actin decreases tropomyosin-based inhibition of actomyosin activity

Recent proteomics studies of vertebrate striated muscle have identified lysine acetylation at several sites on actin. Acetylation is a reversible post-translational modification that neutralizes lysine's positive charge. Positively charged residues on actin, particularly Lys³²⁶ and Lys³²⁸, are predicted to form critical electrostatic interactions with tropomyosin (Tpm) that promote its binding to filamentous (F)-actin and bias Tpm to an azimuthal location where it impedes myosin attachment. The troponin (Tn) complex also influences Tpm's position along F-actin as a function of Ca²⁺ to regulate exposure of myosin-binding sites and, thus, myosin cross-bridge recruitment and force production. Interestingly, Lys³²⁶ and Lys³²⁸ are among the documented acetylated residues. Using an acetic anhydride-based labeling approach, we showed that excessive, nonspecific actin acetylation did not disrupt characteristic F-actin-Tpm binding. However, it significantly reduced Tpm-mediated inhibition of myosin attachment, as reflected by increased F-actin-Tpm motility that persisted in the presence of Tn and submaximal Ca²⁺ Furthermore, decreasing the extent of chemical acetylation, to presumptively target highly reactive Lys³²⁶ and Lys³²⁸, also resulted in less inhibited F-actin-Tpm, implying that modifying only these residues influences Tpm's location and, potentially, thin filament regulation. To unequivocally determine the residue-specific consequences of acetylation on Tn-Tpm-based regulation of actomyosin activity, we assessed the effects of K326Q and K328Q acetyl (Ac)-mimetic actin on Ca²⁺-dependent, in vitro motility parameters of reconstituted thin filaments (RTFs). Incorporation of K328Q actin significantly enhanced Ca²⁺ sensitivity of RTF activation relative to control. Together, our findings suggest that actin acetylation, especially Lys³²⁸, modulates muscle contraction via disrupting inhibitory Tpm positioning.

The role of propofol hydroxyl group in 5-lipoxygenase recognition

Propofol is a clinically important intravenous anesthetic. We previously reported that it directly inhibited 5-lipoxygenase (5-LOX), a key enzyme for leukotriene biosynthesis. Because the hydroxyl group in propofol (propofol 1-hydroxyl) is critical for its anesthetic effect, we examined if its presence would be inevitable for 5-lipoxygenase recognition. Fropofol is developed by substituting the hydroxy group in propofol with fluorine. We found that propofol 1-hydroxyl was important for 5-lipoxygenase recognition, but it was not absolutely necessary. Azi-fropofol bound to 5-LOX at one of the two propofol binding sites of 5-LOX (pocket around Phe-187), suggesting that propofol 1-hydroxyl is important for 5-LOX inhibition at the other propofol binding site (pocket around Val-431). Interestingly, 5-hydroperoxyeicosatetraenoic acid (5-HpETE) production was significantly increased by stimulation with calcium ionophore A23187 in HEK293 cells expressing 5-LOX, suggesting that the fropofol binding site is important for the conversion from 5-HpETE to leukotriene A₄. We also indicated that propofol 1-hydroxyl might have contributed to interaction with wider targets among our body.

Anesthetic Agents Isoflurane and Propofol Decrease Maximal Ca2+-Activated Force and Thus Contractility in the Failing Myocardium

In the normal heart, frequently used anesthetics such as isoflurane and propofol can reduce inotropy. However, the impact of these agents on the failing myocardium is unclear. Here, we examined whether and how isoflurane and propofol influence cardiac contractility in intact cardiac muscles from rats treated with monocrotaline to induce heart failure. We measured force and intracellular Ca2+ ([Ca2 +]i) in trabeculae from the right ventricles of the rats in the absence or presence of propofol or isoflurane. At low to moderate concentrations, both propofol and isoflurane dose-dependently depressed cardiac force generation in failing trabeculae without altering [Ca2+]i At high doses, propofol (but not isoflurane) also decreased amplitude of [Ca2+]i transients. During steady-state activation, both propofol and isoflurane impaired maximal Ca2+-activated force (Fmax) while increasing the amount of [Ca2+]i required for 50% of maximal activation (Ca50). These events occurred without apparent change in the Hill coefficient, suggesting no impairment of cooperativity. Exposing these same muscles to the anesthetics after fiber skinning resulted in a similar decrement in Fmax and rise in Ca50 but no change in the myofibrillar ATPase-Ca2+ relationship. Thus, our study demonstrates that challenging the failing myocardium with commonly used anesthetic agents such as propofol and isoflurane leads to reduced force development as a result of lowered myofilament responsiveness to Ca2+ SIGNIFICANCE STATEMENT: Commonly used anesthetics such as isoflurane and propofol can impair myocardial contractility in subjects with heart failure by lowering myofilament responsiveness to Ca2+. High doses of propofol can also reduce the overall amplitude of the intracellular Ca2+ transient. These findings may have important implications for the safety and quality of intra- and perioperative care of patients with heart failure and other cardiac disorders.

High-dose Propofol Anesthesia Reduces the Occurrence of Postoperative Cognitive Dysfunction via Maintaining Cytoskeleton

Postoperative cognitive dysfunction (POCD) is a common postoperative complication observed in patients following. Here we tested the molecular mechanisms of memory loss in hippocampus of rat POCD model. We found that high-dose propofol anesthesia significantly alleviated spatial memory loss. The proteomes and transcriptomes in hippocampus showed that hippocampal cytoskeleton related pathways were abnormal in low group while not in high group. The protein assays confirmed that hippocampal actin cytoskeleton was depolymerized in low group while maintained in high group. This study confirms that high-dose propofol anesthesia could mitigate the development of POCD and provides evidences for actin cytoskeleton associated with this syndrome.

Inhibition of gasotransmitters production and calcium influx affect cardiodynamic variables and cardiac oxidative stress in propofol-anesthetized male Wistar rats

It has been assumed that the cardioprotective effects of propofol are due to its non-anesthetic pleiotropic cardiac and vasodilator effects, in which gasotransmitters (NO, H2S, and CO) as well as calcium influx could be involved. The study on isolated rat heart was performed using 4 experimental groups (n = 7 in each): (1) bolus injection of propofol (100 mg/kg body mass, i.p.); (2) L-NAME (NO synthase inhibitor, 60 mg/kg body mass, i.p.) + propofol; (3) DL-PAG (H2S synthase inhibitor, 50 mg/kg body mass, i.p.) + propofol; (4) ZnPPIX (CO synthase inhibitor, 50 μmol/kg body mass, i.p.) + propofol. Before and after the verapamil (3 μmol/L) administration, cardiodynamic parameters were recorded (dp/dtmax, dp/dtmin, systolic left ventricular pressure, diastolic left ventricular pressure, heart rate, coronary flow), as well as coronary and cardiac oxidative stress parameters. The results showed significant increases of diastolic left ventricular pressure following NO and CO inhibition, but also increases of coronary flow following H2S and CO inhibition. Following verapamil administration, significant decreases of dp/dtmax were noted after NO and CO inhibition, then increase of diastolic left ventricular pressure following CO inhibition, and increase of coronary flow following NO, H2S, or CO inhibition. Oxidative stress markers were increased but catalase activity was significantly decreased in cardiac tissue. Gasotransmitters and calcium influx are involved in pleiotropic cardiovascular effects of propofol in male Wistar rats.

Total Intravenous Anesthesia Maintained the Degree of Pre-Existing Mitral Regurgitation Better than Isoflurane Anesthesia in Cardiac Surgery: A Randomized Controlled Trial

Accurate assessment of mitral regurgitation (MR) is critical during mitral valve repair surgery. However, anesthesia may influence the degree of mitral regurgitation by changing pre- and after-load or cardiac contractility. Therefore, we compared changes in mitral regurgitation by total intravenous anesthesia (TIVA) and inhalation anesthesia in patients with pre-existing mitral regurgitation. This was a double-blind randomized controlled study conducted at a tertiary care center in 2018. Fifty-four mitral regurgitation patents undergoing elective cardiac surgery were randomly assigned to receive TIVA or isoflurane. Primary endpoint was change of regurgitation volume by anesthesia. The reduction of regurgitation volume by anesthesia was greater in the isoflurane group than in the TIVA group (mean (95% confidence interval CI): -0.20 (-6.15, 5.75) vs. -9.66 (-15.77, -3.56), mL·beat^-1, p = 0.0266) and this phenomenon was more prominent with severe mitral regurgitation (grade 3 or 4) (mean (95% CI): -0.33 (-9.10, 8.44) vs. -16.20 (-24.22, -8.18), mL·beat^-1, p = 0.0079). Among patients with MR grade 3 or 4, 94% remained the same with TIVA during anesthesia compared to 56% with isoflurane. In conclusion, TIVA maintained the pre-anesthetic state of mitral regurgitation relatively well, while the severity of mitral regurgitation tended to decrease with isoflurane anesthesia.

The actin 'A-triad's' role in contractile regulation in health and disease

Striated muscle contraction is regulated by Ca²⁺ -dependent modulation of myosin cross-bridge binding to F-actin by the thin filament troponin (Tn)-tropomyosin (Tm) complex. In the absence of Ca²⁺ , Tn binds to actin and constrains Tm to an azimuthal location where it sterically occludes myosin binding sites along the thin filament surface. This limits force production and promotes muscle relaxation. In addition to Tn-actin interactions, inhibitory Tm positioning requires associations between other thin filament constituents. For example, the actin 'A-triad', composed of residues K326, K328 and R147, forms numerous, highly favourable electrostatic contacts with Tm that are critical for establishing its inhibitory azimuthal binding position. Here, we review recent findings, including the identification and interrogation of modifications within and proximal to the A-triad that are associated with disease and/or altered muscle behaviour, which highlight the surface feature's role in F-actin-Tm interactions and contractile regulation.

Fropofol decreases force development in cardiac muscle

Supranormal contractile properties are frequently associated with cardiac diseases. Anesthetic agents, including propofol, can depress myocardial contraction. We tested the hypothesis that fropofol, a propofol derivative, reduces force development in cardiac muscles via inhibition of cross-bridge cycling and may therefore have therapeutic potential. Force and intracellular Ca²⁺ concentration ([Ca²⁺]_i) transients of rat trabecular muscles were determined. Myofilament ATPase, actin-activated myosin ATPase, and velocity of actin filaments propelled by myosin were also measured. Fropofol dose dependently decreased force without altering [Ca²⁺]_i in normal and pressure-induced hypertrophied-hypercontractile muscles. Similarly, fropofol depressed maximum Ca²⁺-activated force ( F_max) and increased the [Ca²⁺]_i required for 50% of F_max (Ca₅₀) at steady state without affecting the Hill coefficient in both intact and skinned cardiac fibers. The drug also depressed cardiac myofibrillar and actin-activated myosin ATPase activity. In vitro actin sliding velocity was significantly reduced when fropofol was introduced during rigor binding of cross-bridges. The data suggest that the depressing effects of fropofol on cardiac contractility are likely to be related to direct targeting of actomyosin interactions. From a clinical standpoint, these findings are particularly significant, given that fropofol is a nonanesthetic small molecule that decreases myocardial contractility specifically and thus may be useful in the treatment of hypercontractile cardiac disorders.-Ren, X., Schmidt, W., Huang, Y., Lu, H., Liu, W., Bu, W., Eckenhoff, R., Cammarato, A., Gao, W. D. Fropofol decreases force development in cardiac muscle.

Heart Failure-Related Hyperphosphorylation in the Cardiac Troponin I C Terminus Has Divergent Effects on Cardiac Function In Vivo

BACKGROUND

In human heart failure, Ser199 (equivalent to Ser200 in mouse) of cTnI (cardiac troponin I) is significantly hyperphosphorylated, and in vitro studies suggest that it enhances myofilament calcium sensitivity and alters calpain-mediated cTnI proteolysis. However, how its hyperphosphorylation affects cardiac function in vivo remains unknown.

METHODS AND RESULTS

To address the question, 2 transgenic mouse models were generated: a phospho-mimetic cTnIS200D and a phospho-silenced cTnIS200A, each driven by the cardiomyocyte-specific α-myosin heavy chain promoter. Cardiac structure assessed by echocardiography and histology was normal in both transgenic models compared with littermate controls (n=5). Baseline in vivo hemodynamics and isolated muscle studies showed that cTnIS200D significantly prolonged relaxation and lowered left ventricular peak filling rate, whereas ejection fraction and force development were normal (n=5). However, with increased heart rate or β-adrenergic stimulation, cTnIS200D mice had less enhanced ejection fraction or force development versus controls, whereas relaxation improved similarly to controls (n=5). By contrast, cTnIS200A was functionally normal both at baseline and under the physiological stresses. To test whether either mutation impacted cardiac response to ischemic stress, isolated hearts were subjected to ischemia/reperfusion. cTnIS200D were protected, recovering 88±8% of contractile function versus 35±15% in littermate controls and 28±8% in cTnIS200A (n=5). This was associated with less cTnI proteolysis in cTnIS200D hearts.

CONCLUSIONS

Hyperphosphorylation of this serine in cTnI C terminus impacts heart function by depressing diastolic function at baseline and limiting systolic reserve under physiological stresses. However, paradoxically, it preserves heart function after ischemia/reperfusion injury, potentially by decreasing proteolysis of cTnI.

Common general anesthetic propofol impairs kinesin processivity

Propofol is the most widely used i.v. general anesthetic to induce and maintain anesthesia. It is now recognized that this small molecule influences ligand-gated channels, including the GABA_A receptor and others. Specific propofol binding sites have been mapped using photoaffinity ligands and mutagenesis; however, their precise target interaction profiles fail to provide complete mechanistic underpinnings for the anesthetic state. These results suggest that propofol and other common anesthetics, such as etomidate and ketamine, may target additional protein networks of the CNS to contribute to the desired and undesired anesthesia end points. Some evidence for anesthetic interactions with the cytoskeleton exists, but the molecular motors have received no attention as anesthetic targets. We have recently discovered that propofol inhibits conventional kinesin-1 KIF5B and kinesin-2 KIF3AB and KIF3AC, causing a significant reduction in the distances that these processive kinesins can travel. These microtubule-based motors are highly expressed in the CNS and the major anterograde transporters of cargos, such as mitochondria, synaptic vesicle precursors, neurotransmitter receptors, cell signaling and adhesion molecules, and ciliary intraflagellar transport particles. The single-molecule results presented show that the kinesin processive stepping distance decreases 40-60% with EC₅₀ values <100 nM propofol without an effect on velocity. The lack of a velocity effect suggests that propofol is not binding at the ATP site or allosteric sites that modulate microtubule-activated ATP turnover. Rather, we propose that a transient propofol allosteric site forms when the motor head binds to the microtubule during stepping.